Tintin finds a lost briefcase and returns it to the owner, Professor Hector Alembick, who is a sigilographer, an expert on seals. He shows Tintin his collection of seals, including one which belonged to the Syldavian King Ottokar IV. Tintin then discovers that he and Alembick are under surveillance by some strange men. Tintin’s flat is even bombed in an attempt to kill him. Suspecting a Syldavian connection, Tintin offers to accompany Alembick to Syldavia for research.
On the plane Tintin begins to suspect his companion. The Alembick travelling with him doesn’t smoke and doesn’t seem to need the spectacles he wears, while the Alembick he first met smoked heavily and had very poor eyesight. During a layover, Tintin fakes a fall and grabs Alembick’s beard, thinking it is false and Alembick is an imposter. However, it is (for Alembick) painfully real. Tintin decides to let the matter drop but then, while flying over Syldavia, it is the pilot of the plane who opens a trap door and Tintin drops out, landing in a haywagon.
Tintin has a hunch that a plot is afoot to steal the sceptre of King Ottokar IV. In Syldavia, the reigning King must possess the sceptre to rule or he will be forced to abdicate. Every year he rides in a parade during St. Vladimir’s Day carrying it, while the people sing the national anthem. Tintin succeeds in warning the reigning King, Muskar XII, despite the efforts of the conspirators. He and the King rush to the royal treasure room to find Alembick, the royal photographer and some guards unconscious and the sceptre missing.
Tintin’s friends Thomson and Thompson are summoned to investigate but their theory on how the sceptre was stolen proves bad and painful for them. Later on, Tintin notices a spring cannon in a toy shop and this gives him the clue. Professor Alembick had asked for some photographs to be taken of the sceptre, but the camera was a spring cannon in disguise, which allowed him to catapult it out of the castle into a nearby forest.